Photoinitiators used in photo-curable compositions such as coating compositions need to provide good cure speed, low volatility, low yellowing, and good solubility, in particular in coating compositions. Increasing health awareness has furthermore led to lower legal limits in extractables, in particular where photocured coatings are likely to come or are intended to come into contact with food or food ingredients.
Another requirement for the photoinitiators to be useful in practice is that such compounds be accessable via efficient large scale processes and allow for easy processing and long term stability in coating compositions. For the latter, it is desired that the photoinitiators are low viscuous liquids which are highly compatible with other ingredients of photocurable compositions and thus do not give rise to flocculation or similar forms of deposit formation upon longer storage.
Benzophenone is known for its capability to provide fast curing, has a good solubility and shows a low degree of yellowing while being cheap and widely available. The drawbacks associated with its use, however, are an undesireable high degree of extractability and migration as well as its strong odor which prevents it from being used in modern packaging industry.
Another material that has an absorption maximum making it in principle useful as a photoinitiator is 4-hydroxybenzophenone. It is, however, rarely used for that purpose due to its poor solubility in UV curable formulations and its lower reactivity.
Several attempts were made to overcome these drawbacks by using functionalized benzophenones such as benzophenone-2-methyl ester (Speedcure MBB ex Lambson) and acrylated benzophenone (IRR261 ex UCB). Even though the aformentioned compounds are less volatile and thus cause less odor, their activity as photoinitiator is significantly lower compared to benzophenone.
Further known examples include benzophenone derivatives bound to a poly butandiol ether backbone via carboxymethoxy moities which are described in detail in WO 03/033452 A1 and are sold commercially as Omnipol BP.
The synthesis of these products starting from 4-hydroxybenzophenone requires several tedious reaction and isolation steps, thus rendering the overall efficiency low.
The products are also typically wax-like or highly viscous and thus hard to handle. Another disadvantage is that they still exhibit a substantial migration level when applied in coating compositions. Such products are also not compatible with photocurable compositions over the entire desired range and may form deposits upon longer storage.
Another attempt to overcome the problems of migration and odor was made by binding inter alia 4-benzophenonecarboxylic esters to a polymeric backbone bearing several hydroxy groups such as polyethylene glycol. Such compounds are, for example, disclosed in EP 2 394 676 A1 and are commercially available under the tradename Genopol* BP.
The disadvantages of these compounds are, however, that the carboxy substituted benzophenone derivatives have reduced reactivity.
EP 632 329 A and EP 2 617 705 disclose photoinitiators of the benzophenone, acetophenone or hydroxyacetophenone type which were functionalized with diisocyanates. Such products may be further functionalized with hydroxyalkyl(meth)acrylates to obtain photoiniitators which are capable of being incorporated into a polymer upon irradiation.
DE 10 2006 047 863 A discloses Photoinitiators inter alia of the hydroxy benzophenone type which are substituted by various ether functionalities.
WO03/064061 A discloses as two single compounds acrylic esters of ethoxylated 4-hydroxy-benzophenone with chain lengths of n=1 and n=4. The synthesis of such compounds is, however, according to own investigations, tedious and thus is not suitable to allow large scale and/or efficient manufacturing.
DE 102012216170 A discloses photoreactive polymers prepared from a monomer selected from (meth)acrylic esters of ethoxylated 2,5-dimethyl-4-hydroxy-benzophenone with chain lengths of 1 to 30, preferably 5 to 20.
The synthesis of monomers shown in Example 3 of DE 102012216170 A for n=9 proceeds via tosylation of the respective polyhydroxyethylacrylate and the coupling of the activated ester with 2,5-dimethyl-4-hydroxy-benzophenone, and is thus quite inefficient in terms of aiding agents.
The monomers as such were not employed as photoinitiators.
WO99/47176 A discloses a PEG200 ethoxylated 4-hydroxymethyl-benzophenone as starting material for further functionalisation. It is not used as photoinitiator.